Silicon carbide is an attractive material for many applications due to its high strength, excellent thermal stability and resistance to oxidising and chemically corrosive environments.
Example uses of silicon carbide used as a refractory include tiles, bricks and blocks, kiln furniture, linings for high temperature furnaces and crucibles for molten metal. An example of a reaction bonded silicon carbide refractory is disclosed in U.S. Pat. No. 4,990,469. Silicon carbide particles, silicon and an inorganic oxide are processed by heat treatment under nitrogen to provide materials with a density of approximately 2.7 g/cm3 and with bending strengths from 30 MPa to 170 MPa. These early silicon nitride-bonded silicon carbide materials typically contain free silica (SiO2) in the refractory product which is detrimental to the high temperature strength of the silicon carbide. Accordingly, developments of the initial reaction-bonded silicon carbide have used a SiAlON bond phase and in particular β-SiAlON. SiAlONs are compounds based on the silicon nitride (Si3N4) where Si and N atoms are replaced in the lattice with Al and O atoms. Examples SiAlON-bonded silicon carbide materials are described in U.S. Pat. No. 5,521,129 and US 2006/0281625.
There is an increasing requirement for energy saving in the ceramics manufacturing industry, not only to reduce production costs but also to satisfy environmentally friendly criteria. Manufacturers of ceramic products, ranging from porcelain through technical ceramics to building materials, are now using methods to improve efficiency by automatically loading and unloading kiln cars and using faster firing cycles. The kiln furniture used for these applications must be low mass and thermally stable, have a high strength, have excellent thermal shock and oxidation resistance, the ability to withstand high temperatures and retain strength at operating temperatures.
In addition, there is an increasing use of high strength, wear resistant ceramic products in a number of more exotic applications; including aircraft engines, brake discs for high performance cars, cyclones and hydro cyclones as well as for personal body armour. All of these applications require ceramic products having high strength, high hardness, excellent impact resistance, excellent abrasion resistance (for certain applications) and the ability to withstand sudden temperatures changes.
Typically, the established silicon nitride-bonded silicon carbides for refractory applications are shape processed by hydraulic, hammer or vibration pressing, or extrusion. The resulting shapes are fired in a nitrogen atmosphere to convert silicon metal powder to α-silicon nitride (Si3N4) and β-silicon nitride (the bonding system) in situ. The silicon metal powder used for this type of product frequently contains 96-98% silicon and iron present, as an impurity. The iron is known to act as a catalyst during the metal-nitrogen (gas/solid) reaction. It is well known that all silicon carbide refractory shapes are susceptible to oxidation above about 1400° C., which eventually leads to product failure. This includes silicon nitride-bonded silicon carbide refractories.
For certain shapes made by slip casting or vibration casting, clay is added to the composition to aid the casting process, which contains silica, alumina and a number of other oxides. During the firing process in a nitrogen atmosphere, these combine with the silicon metal to form a bond containing predominantly silicon oxynitride as well as some α-silicon nitride, β-silicon nitride and glassy phases. Silicon oxynitride and the glassy phases significantly improve the cold abrasion resistance but are detrimental to the thermal properties, particularly resistance to thermal shock and oxidation.
Accordingly, there is a need for a SiAlON bonded silicon carbide material that has improved oxidation resistance, higher strength at ambient and high temperature and which has excellent thermal shock and abrasion properties.